The present invention relates generally to an electromagnetic radiation detector, and more particularly to a highly sensitive optical fiber infrared radiation detector.
Devices for detecting electromagnetic radiation have a variety of applications. Infrared detectors in particular have applications in surveillance, weather forecasting and medical diagnostics. Many excellent detectors exist for the ultraviolet-to-near-infrared portion of the electromagnetic spectrum, including photomultiplier tubes and silicon and gallium arsenide detectors. Longer wavelength radiation, such as infrared, is more difficult to detect with a high degree of sensitivity because it is more easily lost in the noise caused by local temperature fluctuations. The more sensitive devices currently available are photon detectors which employ semiconductor materials like gold-doped germanium and mercury cadmium telluride. These materials are quite expensive and require operation at liquid nitrogen temperature (around 77.degree. K.), which presents additional cost and complexity. Also, these devices are too large because of the associated cooling apparatus to be used for some applications, such as incorporation into a missile. These devices are also not capable of operating over a broad wavelength range, which is desirable for some applications. Pyroelectric or thermistor devices are currently available for use at room temperature, but these devices are one to two orders of magnitude less sensitive than photon detectors. Also, these devices may not be rugged enough to be used in hostile environments, such as caustic or high-electric-potential environments. Currently, no highly sensitive infrared detector exists which will operate at room temperature.
Optical waveguides or fibers are currently used in interferometers to sense physical parameters by directing light through an optical fiber which is adapted to be altered by the physical parameter, and measuring the change in the phase of the light caused by the physical parameter by measuring the interference pattern created by combining the phase-changed light with light from a reference optical fiber. Examples of such interferometers include Mach-Zehnder and Michelson interferometers. In many of these arrangements, the physical parameter in some way varies the fiber's length and/or refractive index and hence its optical pathlength. This change in optical path length shifts the phase of the light beam passing through the fiber, which shift can be measured by measuring the resulting change in the interferometer's interference pattern. No such device exists to detect infrared radiation.